1. Field of the Invention
The present invention relates generally to an antenna device using a planar or other antenna, and, more particularly, to an antenna device for changing antenna directivity depending on the inclination of the enclosure, etc. of a radio communications apparatus such as a mobile communications terminal apparatus, as well as to a directivity control method and the communications apparatus.
2. Description of the Related Art
While, among recent mobile communications terminal apparatuses, those incorporating a GPS (Global Positioning System) antenna and GPS receiver and performing positioning by receiving radio wave from GPS satellite have become widespread, performance enhancement of the incorporated antenna is hoped for in order to improve positioning accuracy.
Conventional GPS planar antenna incorporated in a mobile communications terminal apparatus will be described with reference to FIG. 1. A mobile terminal 2, equipped with first and second main body portions 4 and 6, has the main body portions 4 and 6 joined with a hinge portion 8 to allow opening and closing. That is, if the main body portion 4 is a fixed portion, the other main body 6 is a movable portion. There is provided a communications antenna 10 on the side of the main body portion 4, whereas there is provided a GPS planar antenna 12 on the side of the main body portion 6. The planar antenna 12 is mounted on a ground pattern portion 16, a grounded conductor on the surface portion of a printed board 14 incorporated in the main body portion 6. The ground pattern portion 16 is provided on one side of the printed board 14 so as to cover the entire surface thereof. In this case, there is mounted a display device 18 on the rear side of the printed board.
Thus, in the case of the planar antenna 12 provided on the main body portion 6, an antenna radiation pattern 20 is formed having a central axis in the orthogonal direction to the surface of the ground pattern portion 16. Gain is high in the direction shown by an arrow 22 having the antenna radiation pattern 20 at the center, allowing radio wave to be readily radiated. On the other hand, gain tends to be low on the rear side of the planar antenna 12 and on the upper (U) and lower (B) sides of the main body portion 6.
The planar antenna 12 is used in the mobile terminal 2 as GPS receive antenna because GPS radio wave transmitted from satellites is circular polarized—a system difficult to be dependent on antenna reception angle—and circular polarized antenna is advantageous for GPS radio wave reception, allowing the planar antenna 12 to be highly efficient and provide high gain.
However, the circular polarized planar antenna 12 is highly directive with radiation pattern concentrated in a specific direction, resulting in gain in the opposite direction tending to be low. If the planar antenna 12 is provided in the mobile terminal 2 or other, angular change with respect to the incoming direction of radio wave affects positioning accuracy. That is, since the planar antenna 12 is remarkably directive, there are angles in which radio wave is strongly and properly received and those in which radio wave is difficult to receive, resulting in low reception sensitivity and deteriorated positioning accuracy depending on the angle during use.
Among patent documents related to such an antenna are Japanese Patent Application Laid-Open Publication Nos. 08-279711 and 10-190347.
Japanese Patent Application Laid-Open Publication No. 08-279711 discloses an antenna device that requires no vertical plane directivity adjustment by automatically pointing the beam in a specific direction irrespective of the usage condition of the mobile terminal. The antenna device comprises, in an antenna device attached to a mobile terminal, an array antenna attached to the mobile terminal cover, phase shifting means connected to the array antenna for adjusting the antenna beam direction and angle detection means connected to the phase shifting means for detecting the angle formed between the mobile terminal main body and cover and is configured to change the phase shift of the phase shifting means depending on the detection results of the phase detection means so as to adjust the antenna beam to a desired direction.
Japanese Patent Application Laid-Open Publication No. 10-190347 discloses a patch antenna device capable of handling a plurality of frequencies. The patch antenna device has a conductive member on the surface of a dielectric substrate shaped into a basic patch portion and additional patch portions. The anode of a PIN diode is connected to one of the patch portions, whereas the cathode is connected to the other patch portion, thus electrically isolating the patch portions from each other when no control DC voltage is imparted to the diode. When control DC voltage is imparted to the diode such that a forward current flows through the diode, the patch portions are electrically connected, resulting in the effective magnitude of the antenna element becoming f2, lower than a resonance frequency f1 when DC voltage is not imparted and showing that the apparatus can handle two frequencies.
Incidentally, the GPS planar antenna 12, provided in the main body portion 6 of the mobile terminal 2 shown in FIG. 1, has its gain biased in a specific direction. While the receive level of incoming signal from the direction opposite to the arrow 22 is high with high positioning accuracy, the receive level of incoming signal from any other directions—left, upper (U) and lower (B) directions in the figure—is low, with low positioning accuracy. There is a possibility that, if the mobile terminal 2 equipped with such an antenna is put in a chest pocket of the user's clothing with the antenna 10 pointing up in the zenith direction, positioning accuracy may degrade due to low directivity of the planar antenna 12 toward GPS signal arriving from the zenith direction. To properly receive GPS signal from the zenith direction, the planar antenna 12 must be set up in the zenith direction. Thus, relative angular change between the circular polarized planar antenna 12 and a GPS satellite in the zenith direction affects positioning accuracy. GPS radio wave reception is carried out irrespective of the angle of the planar antenna 12, including regular and automatic acquisition of position information and position search for the user carrying a mobile terminal with built-in GPS from other party. For this reason, proper GPS radio wave reception demands that directivity of the planar antenna 12 be pointed toward the incoming direction of GPS radio wave so as not to be dependent on the angle between the mobile terminal 2 and the GPS satellite.
The antenna device disclosed by Japanese Patent Application Laid-Open Publication No. 08-279711, an array antenna provided with a plurality of antenna elements, is configured to vary the phase of a signal powering each element depending on the inclination angle of the mobile terminal cover, thus pointing the antenna directivity toward a given direction through combining of electromagnetic wave radiated from each element. Directivity change requires a phase shifting circuit that combines a plurality of PIN diodes and a delay line. Array antenna has a sharp antenna beam and is suited for a point-to-point communication in which a mobile terminal communicates with another because of strong directivity in a specific direction. Antenna gain is extremely low in directions other than the specific direction in which directivity is concentrated, resulting in low positioning accuracy in the case of GPS satellite radio wave reception using such an antenna with sharp beam for positioning because the antenna receives radio wave only from satellites in the specific direction and thus making the choice of this antenna unfit for GPS radio wave reception.
The patch antenna device disclosed in Japanese Patent Application Laid-Open Publication No. 10-190347 has a basic patch portion and a plurality of additional patch portions formed on the surface portion of the dielectric substrate, with the basic and additional patch portions selectively connected, thus changing the resonance frequency through connection of the additional patch portions. Connection of the additional patch portion to the basic patch portion changes the physical area of the antenna element of the patch antenna, thus providing a plurality of resonance frequencies with a single patch antenna. However, even if addition of the additional patch portion to the basic patch portion changes antenna directivity, this also changes the resonance frequency, thus making the patch antenna unfit for GPS radio wave reception of a specific communication frequency. If the resonance frequency changes, communication becomes impossible, thus making positioning impossible.
Neither Japanese Patent Application Laid-Open Publication No. 08-279711 nor Japanese Patent Application Laid-Open Publication No. 10-190347 describes or suggests a problem of antenna directivity change and control without changing resonance frequencies or means for solving the problem.